Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ (932.1)	$ (886.2)	$ (962.2)	$ (840.2)
Net Realized/Unrealized Gains Losses included in Net Income	(0.2)	0.3	7.2	1.0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	8.5	(47.9)	(30.1)	(93.0)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	21.5	17.7	56.0	16.1
Ending Balance	(902.3)	(916.1)	(902.3)	(916.1)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers out of Level 3			32.2
Derivative Liability Recorded at Merger Date	5.4		5.4
The Connecticut Light And Power Company [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	(910.7)	(858.7)	(931.6)	(806.1)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	(2.8)	(47.9)	(23.8)	(92.9)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	22.6	15.5	64.5	7.9
Ending Balance	(890.9)	(891.1)	(890.9)	(891.1)
Western Massachusetts Electric Company [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	(13.5)		(7.3)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	9.8		3.6
Ending Balance	(3.7)		(3.7)
NSTAR Electric Company [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	(15.8)	(1.0)	(3.4)	(2.4)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	1.4	(1.2)	(13.2)	(1.3)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	0.6	0.6	2.8	2.1
Ending Balance	$ (13.8)	$ (1.6)	$ (13.8)	$ (1.6)